1. Field of the Invention
The present invention relates to a radiation-proof resin composition and a radiation-proof cable. In particular, it relates to a radiation-proof resin composition and a radiation-proof cable, which are excellent in water-resistant property.
2. Description of the Related Art
Electric wires and cables used in nuclear power stations, such as boiling water reactors (BWRs), or pressurized water reactors (PWRs), are exposed to heat and radiations during normal running of each nuclear reactor operated in specified running conditions, and also in the event of loss-of-coolant accidents, fires etc. Accordingly, in case of these accidents, electric wires and cables used in the nuclear power stations are required to have high flame retardancy and radiation resistance.
Used as conventional electric wire and cable sheath materials are a polymer material containing chlorine such as polychloroprene rubber (CR), chlorosulfonated polyethylene (CSM), chlorinated polyethylene (CM) etc. so as to provide heat resistance, radiation resistance and flame retardancy while retaining the mechanical property. The CR sheath materials are used mainly for BWRs (heat resistance required: 121° C.×7 days, radiation resistance required: 760 kGy), the CSM sheath materials are used mainly for PWRs (heat resistance required: 140° C.×9 days, radiation resistance required: 2 MGy). Here, when a polymer compound material containing a halogen such as chlorine is exposed to heat and radiations, the halogen is likely to desorb from the material. In this case, the material includes an ionic component containing the halogen.
JP-A-S62-161850 discloses a flame-retardant composition that is composed of vinyl chloride graftmer and 10 to 70 parts by wt of at least one of calcium carbonate and magnesium chloride to 100 parts by wt of the vinyl chloride graftmer, 3 to 50 parts by wt of at least one of baked clay and baked silica, and a flame retarder, and exhibits an oxygen index of 27 or more.
The flame-retardant composition in JP-A-S62-161850 thus composed is excellent in arc spark resistance so that it can be used as a covering material for substrate wiring of various devices.
Also, because as an evaluation method for cable covering materials, it is difficult to simultaneously apply heat and radiations to the cable covering materials because of requiring a special apparatus, the evaluation method uses sequential ageing (irradiation after thermal ageing). Also, there is considered reversed sequential ageing (thermal ageing after irradiation).
If the flame-retardant composition including halogen is exposed to heated water and radiation when assumed loss-of-coolant accidents occur, the ionic component occurred by the halogen desorption reaction absorbs the heated water to be swollen. The flame-retardant composition as disclosed in JP-A-S62-161850 is not intended for use in which radiation resistance is required, and it therefore cannot be used directly as a BWR sheath material or a PWR sheath material. Namely, when a cable using the flame-retardant composition as disclosed in JP-A-S62-161850 as the sheath material is exposed to heated water, heat and radiations, the ionic component occurred in the sheath material may not be captured such that it absorbs water and be swollen to lower the mechanical strength of the sheath material. In this case, the sheath material may be peeled off from the cable.